Vaporizing material for producing highly refractive optical layers

ABSTRACT

The invention relates to a vapour-deposition material for the production of optical layers of high refractive index which comprises titanium oxide and gadolinium oxide and/or dysprosium oxide, to a process for the preparation thereof, and to the use thereof.

The invention relates to a vapour-deposition material for the productionof optical layers of high refractive index which comprises titaniumoxide in a mixture with gadolinium oxide and/or dysprosium oxide.

Optical components are usually provided with thin coatings which areapplied for protection of the surfaces or in order to achieve certainoptical properties.

Optical components of this type are, for example, optical lenses,spectacle lenses, lenses for cameras, binoculars or for other opticalinstruments, beam splitters, prisms, mirrors, window panes and the like.The coatings serve firstly for the treatment of the said surfaces byhardening and/or increasing the chemical resistance in order to reduceor prevent damage by mechanical, chemical or environmental influences,but frequently also secondly to achieve reduced reflection, which is thecase, in particular, for spectacle lenses and camera lenses. Byselecting suitable coating materials, different layer thicknesses andsingle- or multilayered structures comprising, where appropriate,different materials having different refractive indices, it is possibleto achieve a reduction in the reflection to less than 1% over the entirevisible radiation spectrum. By suitable combinations of variousmaterials in suitable layer thicknesses, interference mirrors, beamsplitters, heat filters or cold-light mirrors can also be produced.

In order to produce the above-mentioned coating layers, various, inparticular oxidic materials are known, such as, for example, SiO₂, TiO₂,ZrO₂, MgO, Al₂O₃, but also fluorides, such as MgF₂, and mixtures ofthese substances.

The coating materials are selected here in accordance with the targetoptical properties and in accordance with the processing properties ofthe materials.

The coating of optical substrates is usually carried out using ahigh-vacuum vapour-deposition process. In this, firstly the substrateand a flask containing the vapour-deposition substance are placed in asuitable high-vacuum vapour-deposition apparatus, the apparatus issubsequently evacuated, and the vapour-deposition substance isevaporated by heating and/or electron beam bombardment, with thevapour-deposition material precipitating on the substrate surface in theform of a thin layer. Corresponding apparatuses and processes areconventional prior art.

The choice of starting materials which are suitable for the productionof layers of high refractive index, i.e. having a refractive index of 2or greater, is comparatively limited. Suitable here are essentially theoxides of titanium, zirconium, hafnium and tantalum and mixed oxidesthereof. The most frequently employed is titanium oxide. The layersproduced therefrom are transparent in the visible and near infraredspectral range from about 380 nm to 5 μm and should, like the startingmaterial, have no significant absorption. The refractive index which canbe achieved with titanium oxide at a wavelength of about 500 nm is about2.4. However, absorption is observed on UV irradiation.

In particular in the case of pure titanium(IV) oxide, the risk alsoarises that a loss of oxygen occurs during evaporation, resulting indeposition of sub-stoichiometric titanium oxide layers and thus inlayers which absorb in the visible region. This can be prevented bymeans of suitable measures during the evaporation, such as, for example,the establishment of an oxygen residual pressure, or alternatively bythe addition of certain substances, such as, for example, in accordancewith German Patent 12 28 489, of elements or oxides from the rare-earthgroup. Mention was made here, inter alia, of mixtures of titanium oxidewith praseodymium and/or praseodymium oxide or mixtures of titaniumoxide with cerium and/or cerium oxide.

However, the relative freedom from absorption of these known mixtures isrestricted to the visible spectral region. By contrast, German Patent 1228 489 makes no mention of the ultraviolet or near infrared spectralregion.

A further disadvantage of the pure oxides consists in that theygenerally have high melting and boiling points, which in addition arealso close together. For processing reasons, however, it is advisablefor the vapour-deposition materials to be completely melted beforecommencement of significant evaporation. Only in this way is it possibleto achieve a uniform and adequate evaporation rate, which is necessaryfor the formation of homogeneous layers of uniform thickness. Under theusual working conditions, however, difficulties with respect to meltingoccur, in particular in the case of the oxides of zirconium and hafnium,and also in the case of titanium/zirconium mixed oxides.

The resultant layers are often optically inhomogeneous and, in the caseof the usual multiple application, result in difficulties in thereproducibility of the refractive indices.

For this reason, materials which are intended to lower the melting pointof the metal oxide and are also employed, inter alia, for specificvariation of the refractive indices are also added to the pure metaloxides. However, these materials should be selected so that significantabsorption does not occur in the visible region in the layers formed.

However, it has proven disadvantageous that mixtures of theabove-mentioned metal oxides evaporate incongruently with meltingpoint-lowering additives, i.e. they change their composition in thecourse of the evaporation process and the composition of the depositedlayer is also changed correspondingly.

In order to solve this problem, mixed oxides, such as, for example,La₂Ti₂O₇, have been proposed. However, these give off oxygen, in asimilar way to titanium(IV) oxide, during evaporation, resulting insub-stoichiometric composition of the deposited layers and thus inabsorption phenomena.

In addition to the metal oxides already mentioned above which aresuitable for the production of layers of high refractive index, otherpublications have also already mentioned oxides of the rare-earthmetals.

Thus, for example, mixtures of praseodymium oxide and titanium dioxideare known. These have strong absorption in the spectral region below 400nm and weak absorption in the visible spectral region due to absorptionof the praseodymium ion.

Mixtures of lanthanum oxide and titanium oxide have also already beenproposed a number of times, for example in DE 42 08 811 and in DE 100 65647. However, the lanthanum oxide content results in increasedsensitivity to moisture in the layers produced therewith. In addition,the natural radioactive isotope of lanthanum may cause damage tosensitive optical components due to its radioactive radiation.

U.S. Pat. No. 4,794,607 describes the use of gadolinium oxide layersalternating with aluminium oxide layers which serve as antireflectioncoating for an optical amplifier. Investigations of this material haveshown that homogeneous layers having refractive indices of 1.75 or 1.80,depending on the application method, can be obtained by vapourdeposition with gadolinium oxide (K. Truszkowska, C. Wesolowska,“Optical properties of evaporated gadolinium oxide films in the region0.2-5 μm”, Thin Solid Films (1976), 34(2), 391-4).

The layers produced are thus well away from a target refractive index of2 or greater.

DE-A 3335557 discloses using alternating layers of titanium dioxide andytterbium oxide or layer sequences of aluminium oxide and ytterbiumoxide for the production of antireflection coatings on optical lenses.

It is known that ytterbium oxide can achieve refractive indices of from1.75 to 1.9, depending on the thickness of the applied layers anddepending on the application method. These refractive indices arelikewise not in the high-refractive-index range.

Also known are mixtures of titanium oxide with dysprosium oxide and/orytterbium oxide, but these are, according to WO 95/05670, applied tophotovoltaic cells in order to produce semiconducting metal-oxide layersin the sol/gel process.

No information is given on the achievable refractive indices of thelayers produced.

The object of the present invention was therefore to provide avapour-deposition material for the production of layers having a highrefractive index of at least 2.0 which has high durability, isinsensitive to moisture, acids and alkalis, has low radioactivity, istransparent and non-absorbent over a broad spectral range and does notchange its original composition during melting and evaporation and whichis suitable for the production of layers of high refractive index havingthe same properties.

This object is achieved in accordance with the present invention by avapour-deposition material for the production of optical layers of highrefractive index, comprising titanium oxide and gadolinium oxide and/ordysprosium oxide.

The object according to the invention is furthermore achieved by aprocess for the preparation of a vapour-deposition material for theproduction of optical layers of high refractive index, in which titaniumoxide is mixed with gadolinium oxide and/or dysprosium oxide, themixture is compressed or suspended, shaped and subsequently sintered.

The invention additionally relates to the use of a vapour-depositionmaterial comprising titanium oxide and gadolinium oxide and/ordysprosium oxide for the production of optical layers of high refractiveindex.

In an embodiment according to the invention, the vapour-depositionmaterial comprises titanium dioxide and gadolinium oxide (Gd₂O₃) and/ordysprosium oxide (Dy₂O₃). These materials may be present in a molarratio of from 4:1 to 1:4, where the last number in each case relates tothe molar proportion of gadolinium oxide or dysprosium oxide or to thesum of the molar proportions of gadolinium oxide and dysprosium oxide.

The materials are preferably present in a molar ratio of from 2.6:1 to1:1.3, where again the last number relates to the molar proportion ofgadolinium oxide or dysprosium oxide or to the sum of the molarproportions of gadolinium oxide and dysprosium oxide.

If a mixture of gadolinium oxide and dysprosium oxide is employed, therespective proportion of the individual substances with respect to oneanother is not crucial per se. It can be set in a broad ratio, inparticular in a ratio of from 99:1 to 1:99.

If the refractive indices which can be achieved by coatings with theindividual components are considered, these are from about 2.3 to 2.4 inthe case of coatings with titanium dioxide, while refractive indices offrom about 1.75 to 1.9 can be obtained with pure gadolinium oxide layersand with pure dysprosium oxide layers.

In general, layers having refractive indices which are between the twoindividual values, depending on the molar proportion of the components,can be achieved with mixtures of two components. A relatively high molarproportion of a component having a rather low refractive index wouldaccordingly result in a refractive index of the mixture which is closerto the refractive index of the component of low refractive index than tothe refractive index of the component of relatively high refractiveindex.

It has therefore surprisingly been found that mixtures comprisingtitanium dioxide and gadolinium oxide and/or dysprosium oxide,preferably in the molar proportions described above, enable theproduction of layers having refractive indices which are significantlyhigher than the mixed values to be expected and in particular are closeto the refractive index of pure titanium dioxide. Specifically, layershaving refractive indices of significantly above 2.0, namely in therange from about 2.20 to about 2.30, can be produced with thesemixtures. The defined refractive index desired in each case can be setspecifically within this range via the molar proportion of gadoliniumoxide and/or dysprosium oxide. A relatively high proportion of one orboth of these components then results in a slight lowering of therefractive index obtained.

If both gadolinium oxide and dysprosium oxide are employed in themixture, the respective molar proportion of these two substances doesnot, by contrast, have a significant influence on the refractive indexwhich can be obtained with the titanium dioxide-containing totalmixture. This applies in particular if the sum of the molar proportionsof the mixture is in the above-mentioned limits.

Furthermore, it has advantageously been found that the vapour-depositionmaterials according to the invention evaporate virtually congruently,i.e. their composition does not change significantly over the course ofthe evaporation process. In this way, homogeneous layers having a stablehigh refractive index of n≧2.0, in particular of n≧2.20, can beproduced.

In addition, the optical layers obtained have high transparency in abroad spectral range, i.e. from about 380 nm to 5 μm. In particular,complete transparency can be achieved in the visible and near infraredspectral regions.

Compared with the vapour-deposition materials known from the prior artfor the production of layers of high refractive index, the layersproduced from the vapour-deposition material according to the inventionhave improved durability. This is particularly the case in a moist-warmenvironment, since the vapour-deposition materials according to theinvention do not tend to take up moisture. This property proves to bevery advantageous as early as during preparation of thevapour-deposition materials since special measures for handling andfurther processing of the mixtures are unnecessary. However, the layersof high refractive index formed after deposition on suitable substratesare also particularly stable in a moist-warm environment. Improveddurability to acids and alkalis can likewise be observed.

A further advantage of the vapour-deposition materials of the presentinvention consists in that the substances used have no radioactiveisotopes. Neither the vapour-deposition materials themselves nor thelayers produced therewith therefore emit radioactively, meaning thatsafety measures are unnecessary and damage in this respect to opticalcomponents or detectors which come into contact with the layers is notto be expected.

In a further embodiment of the invention, the vapour-deposition materialfor the production of optical layers of high refractive index comprisestitanium dioxide, titanium and gadolinium oxide (Gd₂O₃) and/ordysprosium oxide (Dy₂O₃).

Due to the addition of metallic titanium, a substoichiometric ratio ofthe mixture with respect to oxygen is set in this case. Release ofoxygen during melting and evaporation of the mixture is thereby avoided.This also improves the handling of the vapour-deposition material, sincethe spitting which otherwise usually occurs during melting andevaporation can be prevented. In this way, the mixture attains aparticularly stable composition, which does not change throughout thesubsequent processing.

Here too, the molar ratio of titanium oxide to gadolinium oxide and/ordysprosium oxide can be from 4:1 to 1:4, but preferably from 2.6:1 to1:1.3, where the last number in each case relates to the molarproportion of gadolinium oxide or dysprosium oxide or to the sum of themolar proportions of gadolinium oxide and/or dysprosium oxide.

In the case where a mixture of gadolinium oxide and dysprosium oxide isused, the proportions of the individual substances with respect to oneanother can be varied within the molar proportion of the two substancesin a ratio of from 99:1 to 1:99.

A mixture of titanium dioxide, titanium and dysprosium oxide has provenparticularly advantageous.

With respect to the parts by weight, the following ratios areadvantageously set: from 50 to 92 parts by weight of gadolinium oxideand/or dysprosium oxide, from 7 to 42 parts by weight of titaniumdioxide and from 0 to 8 parts by weight of titanium, based on the totalweight of the mixture.

Preference is given to the use of from 67 to 76 parts by weight ofgadolinium oxide and/or dysprosium oxide, from 15 to 27 parts by weightof titanium dioxide and from 2 to 5 parts by weight of titanium, basedon the total weight of the mixture.

With respect to the achievable refractive indices of the optical layersobtained with the material, the addition of metallic titanium has nodisadvantageous effects. Here too, refractive indices of greater than2.0 and in particular of from 2.20 to 2.30 can be obtained.

All advantages already described above for the vapour-depositionmaterials according to the invention without addition of titanium alsoapply to the mixtures comprising metallic titanium in addition totitanium dioxide and gadolinium oxide and/or dysprosium oxide.

This means that the mixtures can be processed well, and homogeneouslayers having a stable high refractive index which are non-absorbent andtransparent in a broad spectral range and in addition are stable in amoist-warm environment and do not emit radioactive radiation areobtained therewith.

The vapour-deposition materials according to the invention are preparedby a process in which titanium oxide is mixed with gadolinium oxideand/or dysprosium oxide, the mixture is compressed or suspended, shapedand subsequently sintered.

Titanium oxide and gadolinium oxide and/or dysprosium oxide areadvantageously mixed intimately with one another in a molar ratio offrom 4:1 to 1:4 and in particular in a molar ratio of from 2.6:1 to1:1.3, where the last number in each case relates to the molarproportion of gadolinium oxide or dysprosium oxide or to the sum of themolar proportions of gadolinium oxide and/or dysprosium oxide.

The mixture is compressed and shaped by means of suitable compressionmeasures known per se. However, it is also possible to prepare asuspension of the mixed components in a suitable carrier medium, whichis shaped and subsequently dried. A suitable carrier medium is, forexample, water, to which, if necessary, binders, such as polyvinylalcohol, methyl-cellulose or polyethylene glycol, and, if desired,assistants, such as, for example, wetting agents or anti-foams, areadded. The suspension operation is followed by shaping. In this, variousknown techniques, such as extrusion, injection moulding or spray drying,can be used. The shapes obtained are dried and freed from the binder,for example by burning out. This is carried out for reasons of betterhandling and metering of the mixtures. The shapes into which the mixtureis converted are therefore not limited. Suitable shapes are all thosewhich facilitate simple handling and good metering, which play a specialrole, in particular, in the continuous coating of substrates with thevapour-deposition material according to the invention and thereplenishment process which is necessary for this purpose. Preferredshapes are therefore various tablet shapes, pellets, discs, truncatedcones, grains or granules, rods or also beads.

The shaped mixtures are subsequently sintered. The sintering processhere can be carried out under various conditions. For example, sinteringcan be carried out in air at temperatures of from 1200 to 1600° C.,under an inert gas, such as, for example, argon, at temperatures of from1200 to 1600° C., or under reduced pressure at temperatures of from 1300to 1700° C. and a residual pressure of less than 1 Pa. It isadvantageous here that the sintering process does not, as otherwisegenerally usual, necessarily have to be carried out under reducedpressure. This both reduces the equipment complexity and saves time.

The shaped sintered products formed remain in their shape duringstorage, transport and introduction into the evaporation apparatus andare stable in their composition during the entire subsequent melting andevaporation process.

However, particularly stable vapour-deposition materials are obtained iftitanium dioxide is mixed with titanium, gadolinium oxide and/ordysprosium oxide, compressed or suspended, shaped and subsequentlysintered.

Regarding the molar mixing ratios, the nature of the mouldings and thesintering conditions, no changes arise to the above-mentioned detailsregarding the vapour-deposition materials according to the inventionwithout addition of metallic titanium.

During the mixing, weight ratios of from 50 to 92 parts by weight ofgadolinium oxide and/or dysprosium oxide, from 7 to 42 parts by weightof titanium oxide and from 0 to 8 parts by weight of titanium, based onthe total weight of the mixture, are preferably set. Particularpreference is given to the preparation of a mixture comprising from 67to 76 parts by weight of gadolinium oxide and/or dysprosium oxide, from15 to 27 parts by weight of titanium oxide and from 2 to 5 parts byweight of titanium, based on the total weight of the mixture.

While maintaining the above-mentioned mixing ratios, a mixture ofdysprosium oxide, titanium dioxide and titanium has proven particularlyadvantageous.

After the sintering and cooling, the vapour-deposition materialaccording to the invention is ready for use for the production ofoptical layers of high refractive index having a refractive index ofn≧2.0, in particular n≧2.20.

The vapour-deposition material according to the present invention can beused to coat all suitable substrates, in particular panes, prisms,sheets, shaped substrates, such as optical lenses, spectacle lenses andcamera lenses and the like, which can consist of the known suitablematerials, such as various glasses or plastics. Regarding the nature,size, shape, material and surface quality of the substrates to becoated, the use of the vapour-deposition materials according to theinvention is therefore not subjected to any restrictions at all so longas the substrates can be introduced into the vacuum apparatus and remainstable under the prevailing temperature and pressure conditions.However, it has proven advantageous, in order to increase the density ofthe applied layers, to heat the substrates before and during the coatingoperation, so that the vapour-deposition material hits a pre-heatedsubstrate. Depending on the nature of the substrates employed, they areheated to temperatures of up to 300° C. However, this measure is knownper se.

The vapour-deposition process employed is usually a high-vacuumvapour-deposition process in which the vapour-deposition material isintroduced into a vacuum apparatus in a suitable flask, also known asevaporation crucible or boat, together with the substrate to be coated.

The apparatus is subsequently evacuated, and the vapour-depositionmaterial is caused to evaporate by heating and/or electron beambombardment. The vapour-deposition material precipitates on thesubstrate in the form of a thin layer.

During the evaporation, oxygen is advantageously added in order toensure complete oxidation of the layers. In order to improve theadhesion of the applied layers to, in particular, unheated substrates,the substrate can be bombarded with ions during the coating (ionassisted deposition, plasma assisted deposition).

In general, a plurality of layers, advantageously having alternatelyhigh and low (n≦1.80) refractive indices, are deposited alternately oneon top of the other. In this way, multilayered arrangements are formed,which can provide the substrates coated therewith with, inter alia,greatly reduced reflection. However, multilayered arrangements of thistype on optical substrates have been known per se for some time and arewidely used.

The vapour-deposition materials according to the invention can be usedto produce adherent optical layers of high refractive index on suitablesubstrates which are non-absorbent in a broad spectral range, aretransparent and homogeneous, have a high refractive index of, inparticular, n≧2.20, are stable in a moist-warm environment and to acidsand alkalis and emit no radioactive radiation.

The invention will be explained below by means of a number of examples,but without being restricted thereto.

EXAMPLE 1 Mixture of Dy₂O₃ and TiO₂

68.3% by weight of Dy₂O₃ and 31.7% by weight of titanium dioxide aremixed intimately with one another and subsequently shaped into tablets.These are calcined in air at about 1500° C. for 4 hours. The tablets aresubsequently introduced into a vapour-deposition unit with electron-beamevaporator, for example of the Leybold A700Q type, and evaporated attemperatures of about 2000° C. and an oxygen pressure of 2×10⁻² Pa. Athin layer is deposited on the quartz glasses located in the apparatusas substrates, which had been heated to about 300° C. before the vapourdeposition. The thickness of this layer is set to about 340 nm. Aftercooling and removal from the evaporation apparatus, the transmission andreflection spectra of the coated glasses are measured using aspectrophotometer. The refractive index of the layer is determined fromthe spectra. Homogeneous layers are obtained which have a refractiveindex of 2.28 at a wavelength of 500 nm.

The layers are transparent in the spectral range from 400 nm to about 5μm and have no absorption in this range. The layers are durable in amoist-warm atmosphere, for example at 80% relative atmospheric humidityand 50° C. and have good hardness and adhesive strength.

The coated substrates are boiled in deionised water for 10 minutes andstored for 6 hours at room temperature in a 4.5% by weight salinesolution in deionised water and for 6 hours at room temperature in a0.01 molar hydrochloric acid solution. Detachment of the applied layeris not observed in any of the sample glasses after the respective test.Spots and/or hazing are not evident.

EXAMPLE 2 Mixture of Dy₂O₃, TiO₂ and Ti

Tablets are produced from a homogeneous mixture of 71.03% by weight ofDy₂O₃, 26.20% by weight of titanium dioxide and 2.77% by weight oftitanium and are subsequently calcined under reduced pressure for 10hours at about 1600° C. at a pressure of 1×10⁻¹ Pa.

The tablets are subsequently introduced into a vapour-deposition unitwith electron-beam evaporator of the Leybold A700Q type, and evaporatedat temperatures of about 2000° C. and an oxygen pressure of 2×10⁻² Pa. Athin layer is deposited on the quartz glasses located in the apparatusas substrates, which had been heated to about 300° C. before the vapourdeposition. The thickness of this layer is set to about 340 nm. Aftercooling and removal from the evaporation apparatus, the transmission andreflection spectra of the coated glasses are measured using aspectrophotometer. The refractive index of the layer is determined fromthe spectra. Homogeneous layers are obtained which have a refractiveindex of 2.25 at a wavelength of 500 nm.

The layers are transparent in the spectral range from 400 nm to about 5μm and have no absorption in this range. The layers are durable in amoist-warm atmosphere and have good hardness and adhesive strength.

EXAMPLE 3 Mixture of Dy₂O₃, Gd₂O₃ and TiO₂

25 mol % of Dy₂O₃, 25 mol % of Gd₂O₃ and 50 mol % of titanium dioxideare mixed intimately with one another and subsequently shaped intotablets.

These are calcined in air at about 1500° C. for 6 hours.

The tablets are subsequently introduced into a vapour-deposition unitwith electron-beam evaporator of the Leybold L560 type, and evaporatedat temperatures of about 2000° C. and an oxygen pressure of 2×10⁻² Pa. Athin layer is deposited on the quartz glasses located in the apparatusas substrates, which had been heated to about 300° C. before the vapourdeposition. The thickness of this layer is set to about 240 nm. Aftercooling and removal from the evaporation apparatus, the transmission andreflection spectra of the coated glasses are measured using aspectrophotometer. The refractive index of the layer is determined fromthe spectra. Homogeneous layers are obtained which have a refractiveindex of 2.2 at a wavelength of 500 nm.

The layers are transparent in the spectral range from 400 nm to about 5μm and have no absorption in this range. The layers are durable in amoist-warm atmosphere and have good hardness and adhesive strength.

EXAMPLE 4 Mixture of Gd₂O₃, TiO₂ and Ti

Tablets are produced from a homogeneous mixture of 83.45% by weight ofGd₂O₃, 13.79% by weight of titanium dioxide and 2.76% by weight oftitanium and are subsequently calcined under reduced pressure for 10hours at about 1600° C. at a pressure of 1×10⁻¹ Pa.

The tablets are subsequently introduced into a vapour-deposition unitwith electron-beam evaporator of the Leybold L560 type, and evaporatedat temperatures of about 1800° C. and an oxygen pressure of 2×10⁻² Pa. Athin layer is deposited on the quartz glasses located in the apparatusas substrates, which had been heated to about 300° C. before the vapourdeposition. The thickness of this layer is set to about 220 nm. Aftercooling and removal from the evaporation apparatus, the transmission andreflection spectra of the coated glasses are measured using aspectrophotometer. The refractive index of the layer is determined fromthe spectra. Homogeneous layers are obtained which have a refractiveindex of 2.2 at a wavelength of 500 nm.

The layers are transparent in the spectral range from 400 nm to about 5μm and have no absorption in this range. The layers are durable in amoist-warm atmosphere and have good hardness and adhesive strength.

1. Vapour-deposition material for the production of optical layers ofhigh refractive index, comprising titanium oxide and gadolinium oxideand/or dysprosium oxide.
 2. Vapour-deposition material according toclaim 1, comprising titanium dioxide and gadolinium oxide and/ordysprosium oxide.
 3. Vapour-deposition material according to claim 1,comprising titanium oxide and gadolinium oxide and/or dysprosium oxidein a molar ratio of from 4:1 to 1:4, where the last number in each caserelates to the molar proportion of gadolinium oxide or dysprosium oxideor to the sum of the molar proportions of gadolinium oxide anddysprosium oxide.
 4. Vapour-deposition material according to claim 3,comprising titanium oxide and gadolinium oxide and/or dysprosium oxidein a molar ratio of from 2.6:1 to 1:1.3, where the last number in eachcase relates to the molar proportion of gadolinium oxide or dysprosiumoxide or to the sum of the molar proportions of gadolinium oxide anddysprosium oxide.
 5. Vapour-deposition material according to claim 1,which comprises a mixture of gadolinium oxide and dysprosium oxide in aratio of from 1:99 to 99:1.
 6. Vapour-deposition material according toclaim 1, comprising titanium dioxide, titanium and gadolinium oxideand/or dysprosium oxide.
 7. Vapour-deposition material according toclaim 6, comprising gadolinium oxide and dysprosium oxide in a ratio offrom 1:99 to 99:1.
 8. Vapour-deposition material according to claim 1,comprising from 50 to 92 parts by weight of gadolinium oxide and/ordysprosium oxide, from 7 to 42 parts by weight of titanium dioxide andfrom 0 to 8 parts by weight of titanium, based on the total weight ofthe mixture.
 9. Vapour-deposition material according to claim 8,comprising from 50 to 92 parts by weight of dysprosium oxide, from 7 to42 parts by weight of titanium dioxide and from 0 to 8 parts by weightof titanium, based on the total weight of the mixture. 10.Vapour-deposition material according to claim 1, comprising from 67 to76 parts by weight of gadolinium oxide and/or dysprosium oxide, from 15to 27 parts by weight of titanium dioxide and from 2 to 5 parts byweight of titanium, based on the total weight of the mixture. 11.Vapour-deposition material according to claim 10, comprising from 67 to76 parts by weight of dysprosium oxide, from 15 to 27 parts by weight oftitanium dioxide and from 2 to 5 parts by weight of titanium, based onthe total weight of the mixture.
 12. Process for the preparation of avapour-deposition material according claim 1, in which titanium oxide ismixed with gadolinium oxide and/or dysprosium oxide, the mixture iscompressed or suspended, shaped and subsequently sintered.
 13. Processaccording to claim 12, in which titanium is additionally added to themixture.
 14. Process according to claim 12, in which the sintering iscarried out with inflow of air.
 15. Process according to claim 12, inwhich the sintering is carried out under reduced pressure.
 16. Processaccording to claim 12, in which the sintering is carried out under aninert gas.
 17. Process according to claim 12, in which the mixture isshaped into tablets, panes, pellets, truncated cones, grains, granules,rods or beads.
 18. Use of a vapour-deposition material according toclaim 1 for the production of optical layers of high refractive index.19. Optical layer of high refractive index having a refractive index ofn≧2.0, comprising a vapour-deposition material according to claim
 1. 20.Multilayered optical system comprising at least one optical layer ofhigh refractive index having a refractive index of n≧2.0, comprising avapour-deposition material according to claim 1.